1. Field of the Invention
This invention relates to phase shift keying (PSK) modems and in particular to integrated circuit PSK modems employing field effect transistors and switched capacitor filters, including multi-mode filters having a plurality of selectable capacitors associated therewith.
2. Description of the Prior Art
Discrete component type PSK modems are well known in the prior art. Integrated circuit PSK modems are also known in the prior art, but in those modems, the digital functions are provided on the chip with analog filtering being done externally of the chip.
In the past, in switched capacitor circuits, when a plurality of filter responses was desired, a plurality of filters was provided, each with the desired response.
The modem switch of this invention makes it possible to simply select at least one of a plurality of capacitors to provide a filter with the proper response or to provide other circuits requiring varying values of capacitance with that capacitance, without causing filter skew and other problems in other circuits by stray capacitance added by those capacitors not selected.